The home secretary has been accused of misleading parliament after a high court ruling revealed that unpublished parts of a controversial policy to push back migrant dinghies in the Channel said the tactic would not be used against asylum seekers.

The pushbacks policy was finalised in autumn 2021, yet in January this year Priti Patel said pushing back migrant boats was “absolutely still policy” when she gave evidence to the Lords justice and home affairs committee. She has been accused of giving that evidence even though she knew about the unpublished clauses in the policy not to use pushbacks against asylum seekers.

The former shadow attorney general, Shami Chakrabarti, accused Patel of misleading parliament and called on her to apologise: “This judgment reveals the home secretary connived to mislead refugees, voters and parliament that people expressly seeking asylum could be repelled in UK waters. Priti Patel must apologise and rethink large sections of her borders bill before it returns to the Lords. It is a disgraceful breach of the rule of law.”

Details of the unpublished policy came to light during a legal challenge to the pushbacks plan brought by the Public and Commercial Services Union (PCS), and the NGOs Care4Calais, Channel Rescue and Freedom From Torture.

The Home Office had applied to the high court for public interest immunity to avoid making the details of the pushbacks policy public. This mechanism is used where sensitive issues such as organised crime, terrorism or national security are involved.

But judges said disclosure of the policy did not “give rise to a real risk of serious harm to the public interest”.

The government has always said the pushbacks policy would only be used when safe to deploy it. Restrictions on usage of the tactic are outlined in the nationality and borders bill which is due to return to the House of Lords on 26 April. However, since the policy was announced last October ministers have not said publicly that it would not be used against asylum seekers.

A key part of the unpublished policy disclosed in the high court judgment is that anyone in a dinghy who indicates they wish to claim asylum in the UK should not be pushed back but instead escorted to UK shores. Almost everyone who uses this method to reach the UK is an asylum seeker according to the Home Office’s own data.

The ruling reveals the pushback policy states: “Should a migrant request asylum whilst in UK territorial waters they must be returned to the UK for processing.”

According to the high court judgment, a clause in the unpublished policy says “the actual number of migrant vessels successfully intercepted is likely to be extremely low”. It adds that one of the “acceptable outcomes” is that during operational deployments no migrant vessels are assessed as suitable for safe turnaround.

Paul O’Connor, the head of bargaining at the PCS, said: “PCS has been pressing the Home Office for transparency in these proceedings. They have gone to considerable lengths to keep certain matters shrouded in secrecy. This judgment has left them nowhere to hide and has exposed their real agenda.”

Clare Moseley, the founder of Care4Calais, said: “I’m shocked that this government tried to hide the fact that refugees who request asylum in UK waters have a right to be brought to the UK to process that request.”

Toufique Hossain, the director of public law at Duncan Lewis solicitors, who represents PCS and Care4Calais, described the judgment as “deeply concerning”. “In light of the extreme measures that are now being taken and proposed to prevent asylum claims being considered in the UK, there is a greater need than ever for transparency.”

In a speech on 14 April about tackling illegal migration, the prime minister said Channel pushbacks were unlikely to be used much although provision for the tactic remained in the immigration bill.

“It’s clear that there are extremely limited circumstances when you can safely do this in the English Channel,” Johnson said.

A government spokesperson said: “The entire government is united to protect lives, prevent dangerous Channel crossings and break the business model of criminal gangs profiting from these fatal journeys.

“It is right to consider all safe and legal options to prevent people making these unnecessary journeys, all of which comply with both domestic and international law.”